For the last several decades, air has been the preferred fluid for cooling electronics due to its availability, low cost (cooling fans), and reliable system operation. As chip power densities increased, liquid cooling systems have been introduced. Pioneering work revealed the potential for electronics cooling with microchannels. Liquid cooling in microchannels can be used to dissipate heat fluxes of approximately 1 kW/cm2. However, temperature variation of the chip surface along the coolant stream and large pumping power required for this system are of concern. Flow boiling in microchannels was expected to provide effective cooling without these concerns; however, current research indicates that such high heat fluxes are not projected with current microchannel designs.
Heat transfer in a microchannel or a minichannel is efficient because of the small hydraulic diameters in these channels. Typical channel hydraulic diameters range from 200 micrometers to 3 mm for minichannels and below 200 micrometers for microchannels. The small hydraulic diameters also give rise to a high pressure drop. The increased pressure drop leads to higher pumping power, increased fluid pressure, and a steep pressure gradient in the channels along the flow direction.
Enhancing the channels with surface features, such as roughness, corrugations, turbulators, flow disruptors and fins provide further enhancement in heat transfer, but these features also increase the pressure drop.
For both single-phase flow and flow boiling applications, there is a need for heat transfer enhancement strategies that do not result in significant increase in the pressure drop. Alternatively, there is a need for heat transfer enhancement strategies that increase heat transfer for a given pressure drop or a given equipment size in single-phase flow and two-phase flow including flow boiling and flow condensation. In the case of a two-phase flow, there is additional need to provide a stable flow. In the case of flow boiling, high critical heat flux (CHF), high heat transfer coefficient, and low pressure drop are desired.